they are supposed to figure out how far to drop the body based upon weight in order to avoid decapitation. They probably added a few feet to the rope knowing this.
The "Long drop" method of hanging.
In 1872, William Marwood introduced the "long drop" to Britain for the execution of Frederick Horry at Lincoln prison, as a scientifically worked out way of giving the prisoner a humane death. The concept was invented by doctors in Ireland and Marwood had read about their theory. Longer drops were in use elsewhere by the 1850's, but the short drop had been universal in Britain prior to this time and continued to be used until 1877 when Thomas Askern hanged John Henry Johnson at Armley prison Leeds, on the 3rd of April.
The long drop method was designed to break the prisoner’s neck by allowing them to fall a pre-determined distance and then be brought up with a sharp jerk by the rope. At the end of the drop, the body is still accelerating under the force of gravity but the head is constrained by the noose which delivers a massive blow to the back and one side of the neck, which combined with the downward momentum of the body, breaks the neck and ruptures the spinal cord causing instant deep unconsciousness and rapid death. The later use of the brass eyelet in the noose tended to break the neck with more certainty. Due to its position under the angle of the left jaw, the head is snapped backward with such force that the posterior aspect of the foramen magnum cuts the spinal cord superior to the top vertebra and just a little inferior to the brain stem.
The accurately measured and worked out drop removed most of the prisoner's physical suffering and made the whole process far less traumatic for the officials who now had to witness it in the confines of the execution cell instead of in the open air.
The drop given in the 19th century was usually between 4 and 10 feet, depending on the weight and strength of the prisoner. The weight used to calculate the correct drop is that of the prisoner's body. Up to 1892, the length of drop was calculated to provide a final "striking" force of approximately 1,260 lbs. force which combined with the positioning of the eyelet caused fracture and dislocation of the neck, usually at the 2nd and 3rd or 4th and 5th cervical vertebrae. This is the classic "hangman's fracture". The length of the drop was worked out by the formula 1,260 foot pounds divided by the body weight of the prisoner in pounds = drop in feet. Between 1892 and 1913, a shorter length of drop was used, probably to avoid the decapitation and near decapitations that had occurred with the old table. After 1913, other factors were also taken into account and the drop was calculated to give a final "striking" force of around 1,000 lbs. The Home Office issued a rule restricting all drops to between 5 and 8 feet as this had been found to be an adequate range. The American Military manual also specifies a similar range for prisoners of between 120 and 200 lbs. body weight. In Britain, the drop was worked out and set to the nearest quarter of an inch (see below) to ensure the desired outcome.
In the late 19th century, there was a considerable amount of experimentation to determine the exact amount of drop and James Berry, who succeeded Marwood, had an unfortunate experience when hanging a murderer called Robert Goodale on the 30th of November 1885, who was decapitated by the force of the drop and of Moses Shrimpton who very nearly was. Where the drop was inadequate, the prisoner still strangled to death.
In 1887, Lord Aberdare was commissioned to report into hanging in Britain after these incidents and the unsuccessful attempt to hang John Lee on the 23rd of February 1885, because the trap would not open (he was reprieved after 3 attempts to execute him.) The Aberdare Committee heard a lot of medical evidence and one witness, Dr. Marshall, described a hanging in 1886 as follows.
"I descended immediately into the pit where I found the pulse beating at the rate of 80 to the minute, the wretched man struggling desperately to get his hands and arms free. I came to this conclusion from the intense muscular action in the arms, fore arms and hands, contractions, not continuous but spasmodic, not repeated with any regularity but renewed in different directions and with desperation. From these signs I did not anticipate a placid expression on the face and I regret to say my fears were correct. On removing the white cap about 1 1/2 minutes after the fall I found the eyes starting from the sockets and the tongue protruded, the face exhibiting unmistakable evidence of intense agony."
It is notable that there were quite few problems with early lethal injections before the learning curve was surmounted.